In the most current catalytic reactors, use is made of a solid catalytic bed. In such a catalyst bed, porous bodies are poured or stacked.
In order to avoid an undesirably high pressure drop across the catalyst bed, the bodies or particles used preferably have a size of at least 0.3 mm. This minimum size of the catalyst bodies is necessary to keep the pressure drop which occurs when a stream of reactants is passed through the catalyst bed within technically acceptable limits. In addition to the dimension at lower limit dictated by the permissible pressure drop, the required activity of the catalyst imposes an upper limit upon the dimensions of the catalytically active particles. The high activity required for a number of types of technical catalysts can mostly only be achieved with an area of the active phase of 25 to 500 m.sup.2 per ml. Surface areas of such an order of magnitude are only possible with very small particles, for example, with particles of 0.05 micron.
As a liquid or gas mixture cannot flow through particles of such dimensions, the minute primary particles must be formed into highly-porous bodies having dimensions of at least about 0.3 mm, which may have a large catalytic surface area. An important aim in the production of technical catalysts is to combine the required high porosity with a sufficiently high mechanical strength. The catalyst bodies must not disintegrate during the filling of the reactor and when subjected to sudden temperature differences ("thermal shock").
Under the conditions of the thermal pre-treatment and/or the catalytic reaction to be carried out, almost all catalytically active materials are rapidly sintered into large conglomerates with a negligible small active surface area. Generally speaking, therefore, the active component is applied (in finely-divided form) to a so-called carrier. This carrier has the required thermal stability and is hardly, if at all, sintered at high temperatures. Carriers mostly used are silicon dioxide, aluminium oxide or activated carbon.
A great many technical reactions are characterized by a more or less large, positive or negative, heat effect. In order that chemical reactions may proceed in the desired manner, an efficient supply or removal of the reaction heat is indispensable. Thus with some exothermic reactions, the thermodynamic equilibrium is shifted in an undesirable direction when the temperature is increased. Examples are the synthesis of ammonia and methanol, the oxidation of sulfur dioxide to sulfur trioxide in the production of sulfuric acid, the reaction of sulfur dioxide with hydrogen sulfide in the Claus process, the selective oxidation of H.sub.2 S to S, and the reaction of carbon monoxide with hydrogen to form methane. As thermal energy is generated as these reactions proceed, the temperature of the reaction mixture will rise, and the thermodynamic equilibrium be shifted in an adverse way, unless the reaction heat generated is rapidly and efficiently removed from the reactor.
In endothermic reactions, there may also be a shift of the thermodynamic equilibrium in an undesirable direction, now by absorbing thermal energy. Examples are the methane-steam reforming and the dehydrogenation of ethyl benzene to styrene. Also, the reaction velocity may become so low that the desired reaction is not completed any longer.
In addition to a shift of the thermodynamic equilibrium in an undesirable direction, a change in temperature may adversely affect the selectivity of catalytic reactions.
Examples of reactions in which the temperature affects selectivity are the production of ethylene oxide from ethylene (the undesirable reaction is the formation of water and carbon dioxide), the selective oxidation of hydrogen sulfide to elemental sulfur (the undesirable reaction is the formation of SO.sub.2), and the Fischer Tropsch synthesis. In all cases the temperature is increased as a result of the generation of reaction heat. When the rise in temperature is not prevented by rapidly removing the reaction heat, the selectivity decreases substantially.
Although, as the above examples show, there is a great need of a rapid supply or removal of thermal energy in catalytic reactors, a solid catalyst bed has a poor thermal conductivity. According to the present state of the art, it is virtually impossible to supply thermal energy to, or remove energy from, a solid catalyst bed in an efficient manner. Indeed, this is apparent from the way in which technical reactions are carried out in solid catalyst beds.
It is possible that, in an exothermic reaction, a rise in temperature only leads to a shift of the thermodynamic equilibrium in an undesirable direction, without an intolerable reduction in selectivity. In that case the reaction may be permitted to proceed adiabatically in a solid catalyst bed; naturally, this is only possible with exothermic reactions. After the passage through the reactor, the stream of reactants is cooled in a separate heat exchanger. As the conversion of the reactants is now thermodynamically limited owing to the rise in temperature in the reactor, the reactants must be re-reacted after cooling. The reaction product may be separated and the reactants recycled through the solid catalyst bed. This is effected, for example, in the synthesis of ammonia and in the methanol synthesis. If the reaction product cannot be readily separated, the heat exchanger should be followed by a second solid-bed reactor with heat exchanger. This is the case, for example, in the oxidation of sulfur dioxide to sulfur trioxide. Sometimes, in order to prevent the emission of harmful compounds, even a third reactor with heat exchanger is required. If the series-connection of a plurality of reactors and heat exchangers is not quite possible, and the separation of the reaction product is not possible either, the reaction product is sometimes recycled through the catalyst bed. In that case, so little of the reactants is added to the circulating reaction product per passage through the reactor that these are completely converted. As the rise in temperature must then be well controlled, only a small amount can be converted per pass through the reactor. In cases where the reaction must be carried out at greatly increased pressure, the problems with the supply or removal of reaction heat are even greater. In the synthesis of ammonia and the methanol synthesis, a catalyst bed is used in which reactants are injected at different points at a relatively low temperature.
Such a performance of the process, in which gas streams must be passed through high-pressure reactors in a complicated manner, naturally requires high investments.
It will be clear that none of the above-discussed technical solutions is attractive. Generally speaking, expensive equipment is required, while recycling and separation of reaction products present in low concentration require much energy. This is why, in the cases discussed, fluidized beds are often used. In a fluidized bed, the transport of thermal energy is much easier. In a fluidized bed, the catalyst to be used must satisfy very severe demands as regards mechanical strength and wear resistance, which is by no means possible with every catalyst. Finally, owing to the inevitable wear, catalyst consumption in a fluidized bed is relatively high. Indeed, in many cases it will not be possible to use a fluidized bed.
There are cases in which neither a fluidized bed nor an adiabatic reactor can be used. This is especially true of highly endothermic reactions and reactions in which selectivity is prohibitively decreased when the temperature rises. Examples are the methane-steam reforming and the selective oxidation of ethylene to ethylene oxide. In a selective oxidation of ethylene, a very large heat exchange surface area is applied by using a reactor with no fewer than 20,000 long tubes. In the methane-steam reforming process, it is endeavoured to optimize the supply of heat by adapting the dimensions and shape of the catalyst bodies. In this latter reaction, too, a large number of expensive pipes must be used in the reactor.
In certain cases, the reaction can be conducted with a catalyst suspended in a liquid with a suitable boiling point. The reaction heat is now dissipated by evaporating the liquid. In endothermic reactions, thermal energy can be supplied to the reaction system through the liquid phase. Technically, however, conducting the catalytic reaction with a catalyst suspended in a liquid phase is possible in a few cases only.
It has also been proposed to apply the catalyst exclusively to the wall of the reactor. This is the case, for example, in the performance of the Fischer Tropsch reaction, in which higher hydrocarbons are produced from a mixture of hydrogen and carbon monoxide. The catalyst applied to the wall ensures a good transfer of heat from the catalyst to the environment. One method proposed for applying the catalyst to the wall is the following. The catalyst is applied to the wall as a Raney metal, an alloy of the active metal and aluminium. After the application, the catalyst is activated by dissolving the aluminium with lye. The greater part of the reactor volume is empty, so that the contact of the reactants with the catalytically active surface is poor, and the conversion per pass through the reactor is greatly limited. For this reason the reactants must be recycled through the reactor many times.
In a number of technically important cases, the pressure drop during the passage of the reactants through the catalyst bed must be very small. This applies, for example, to reactors in which flue gas from large plants must be purified, such as in the catalytic removal of nitrogen oxides from flue gas. As a flue gas stream is generally a huge quantity, a proper pressure drop requires a great deal of mechanical energy. The same applies to the purification of automobile exhaust gases. In that case, too, a high pressure drop is impermissible.
At the present time, the only possibility of achieving an acceptable pressure drop, without unduly reducing the contact with the catalyst, is the use of catalysts applied to a honeycomb. For this purpose almost exclusively ceramic honeycombs are used, sometimes referred to a "monoliths", to which the catalytically active material has been applied. However, these ceramic honeycombs are very expensive, and their use is therefore not very attractive.
In a variant of the method in which the catalyst is applied to the wall only, monoliths built up from thin metal plates are used. Such a reactor is made, for example, by rolling up a combination of corrugated and flat thin metal plates and subsequently fixing this assembly by welding. Also, the flat plates may be stacked to form a system with a large number of channels. The catalyst is then applied to the wall of the channels thus produced. Here again, with a normal throughput, conversion is limited, because a relatively large fraction of the reactants passes the catalyst unreacted, or because the reactants require a relatively long residence time in the reactor for a sufficiently high conversion to be achieved.
As stated before, the thermal conduction in a solid catalyst bed is poor. This has been attributed to the low thermal conductivity of the high-porosity carriers supporting the catalytically active material. This is why Kovalanko, O. N. et al, Chemical Abstracts 97 (18) 151409u, have proposed to improve the thermal conduction by increasing the conductivity of the catalyst bodies. They did this by using porous metal bodies as the catalyst carrier. Now, Satterfield has already described that the thermal conductivity of a stack of porous bodies is determined not so much by the conductivity of the material of the bodies, but by the mutual contacts between the bodies (C. N. Satterfield, "Mass Transfer in Heterogeneous Catalysis", MIT Press, Cambridge, Mass., USA (1970), page 173). Measurements by the present inventors have shown that the thermal conductivity of catalyst bodies indeed does not substantially affect the transfer of heat in a catalyst bed.
In WO-A 86/02016, a reactor is described which includes a catalyst carrying reaction bed which consists of sintered metal particles which are in a good heat conducting connection with the reactor wall, which wall is provided on the outside with sintered metal particles for the discharge of reaction heat. Furthermore, a change in phase takes place on the outside of the reactor. It is found that such a reactor system is capable of realizing a discharge of large amounts of heat, but it has the disadvantage that effective adjustment and/or control of the reaction is impossible, or very difficult. This appears, inter alia, from the example, which describes the catalytic combustion of a combustible gas with a calorific value of 35,530 kJ. This should take place at a temperature of 350.degree. C. Owing to the reactor being cooled with evaporating water (steam production) at 110.degree. C., however, the entire reactor is cooled to 110.degree. C., so that the reaction will fail to proceed.
There is accordingly a need for a system in which a chemical reaction, in particular one with a substantial heat effect, can be carried out in a simple manner.